1. Field of the Invention
This invention relates generally to thyristor firing devices and more particularly to thyristor power controller systems with an adjustable firing angle.
2. Description of the Prior Art
For many power applications, it is desirable to be able to vary the current delivered to a load. For this purpose, a solid state switching device, such as a thyristor, is often used.
Commonly, two thyristors are used, each switching on alternate half cycles of an AC current. Power control is accomplished by turning on, or triggering, the thyristors at various times during their half cycle of operation. More power is delivered to a load when a thyristor is triggered early in a half cycle and less is delivered if it is triggered late in a half cycle. The time between the beginning of a particular half cycle and a triggering signal to a thyristor acting on that half cycle is known as the phase, or firing, angle and may be measured in degrees. A firing angle of zero degrees would supply the full current of a half cycle to a load, while a firing angle of 180.degree. would supply no current to a load.
Power controller circuits as described above are well known in the prior art and are exemplified by U.S. Pat. Nos. 3,507,096; 3,577,708; 3,648,437 and 3,745,749.
As an illustrative example, U.S. Pat. No. 3,577,708 teaches a power controller with a magnetic amplifier design including an inductor, a transformer and a thyristor network which varies the firing angle of the thyristors to control the current delivered to a load, which in this case is an electrostatic precipitator.
Thyristor power controllers are capable of efficiently varying the current delivered to devices with very high power ratings, such as electrostatic precipitators. A problem with thyristor controllers in the prior art is that the low power timing and firing angle circuits are affected by transients created by the high power, thyristor firing, circuits.
A problem with using magnetic amplifiers to accomplish phase control is that they usually require a few periods to effect a change in the firing angle. Such slow response time creates problems in some power applications, as in the case of electrostatic precipitators, which require virtually immediate response time in order to suppress sparking and/or arcing. Special costly circuits are often necessary to overcome the slow response time of magnetic amplifier circuits.
A further disadvantage with using magnetic amplifiers for phase control is that the firing angle, and consequently the current controlled by the thyristors, is necessarily a nonlinear function of the control signal. Obviously, this introduces difficulties in calibration and operation.
Yet another disadvantage of magnetic amplifiers is that they produce a distorted sine wave at full current rating. This may be undesirable for some applications where harmonics and transient signals must be kept to a minimum.